3.2 Design for hot-arid zones

The main points:

· Provide maximum shading of
direct and reflected sun radiation in the hot season.· Balance the extremes of summer and winter by movable
parts.· Provide ventilation by regulated air
movement and small openings.· Avoid large
exposed exterior surfaces.· Use reflective
outer surfaces.· Balance the extremes of day
and night temperatures by adequate thermal storage mass· Reduce internal heat production and conduction gain
in hot seasons.· Promote evaporation and
heat loss by radiation.· Increase air
circulation in humid maritime regions.

3.2.1 Climate and design in general(also see Chapter 3.1,
General guidelines)

Climatic condition

The climate of hot-dry zones is in general characterized by high
temperatures (40 - 50°C in summer), with sharp variations in both diurnal
(day / night) and seasonal (summer-/ winter) temperatures; and precipitation
(rainfall, snow) which is scarce, irregular and unreliable, but may nevertheless
cause severe floods. Cold winds and dust/sandstorms prevail in winter. The solar
radiation intensity is high and enhanced by the radiation reflected from the
ground. The air humidity is low and this climate is generally healthier than
those of warm-humid lands. Different climatic zones can be distinguished within
desert regions according to their specific geographical characteristics.
Particular conditions in maritime desert regions mean that the high humidity
causes definite discomfort in summer. On the other hand, the humidity tends to
reduce diurnal variations and moderate temperatures. (also see Chapter 2.2)

Design objectives and response

The main goal of climatic design, on a macro (settlement) and
micro (building) level, is hence to reduce uncomfortable conditions created by
extremes of heat and dryness. Buildings must be adapted to extreme summer /
winter and day / night conditions to achieve a well balanced indoor climate. Not
only cooling is needed; passive heating may also be needed in winter and during
cold nights. Protection is required from the intense radiation from the sun,
ground and surrounding buildings, from dust, sandstorms and insects (flies).
Glare has to be reduced and dust penetration prevented. Settlements and
buildings, therefore, have to be compact, providing shade and controllable
ventilation.

In maritime desert regions, the high humidity requires more air
circulation (ventilation) in summer. It is difficult to design buildings for
this climate.

General remarks

Hot-arid zones or desert regions with scarce vegetation and
saline soils are distributed throughout the world. 15 percent of the
worlds population lives in arid zones; 1/3 of the worlds land mass
and 22% of all potential arable land lies in the arid zone. Most of the
worlds energy reserves (oil) are within or adjacent to these zones. [ 112
]

In the last half century, technological changes have had a major
impact on urban forms and housing throughout the world. The introduction of the
car into the settlements has also drastically altered the traditional urban
pattern of hot-arid regions. The new wide streets reduce the potential for
shading. In addition, the great amount of heat-discharging air conditioners and
large paved surfaces have contributed to changes in the microclimate of urban
situations. Moreover, a change in lifestyles and means of livelihood has
occurred. Mud or adobe buildings, dark interior spaces (very few and small
windows) and sleeping on the roof are probably no longer acceptable to society
in general, but still reality for low income groups. In addition, the proper
handling of climatization devices properly, and the limitations of passive means
are problems which should not be neglected. (also see Chapter 3.1.1)

3.2.2 Settlement Planning (also see Chapter 3.1.2)

The main points:

· Topography, to enhance the
efficiency of passive means · Orientation,
to reduce the sun exposure in summer · Air
movement, to provide ample ventilation in summer and protect from winds in
winter · Form, to design compact settlements
for mutual protection · Hazards, to avoid
dangerous sites

The positioning of settlements can help to take advantage of
local features to improve the micro-climate with regard to comfort. Attention
has to be paid to the topographical altitude, the geomorphology and the most
suitable orientation regarding sun exposure and prevailing winds.
Differentiation must be made between locations on top of hills, on slopes, in
valleys, on flatlands and near water. (see Fig 3/1 to Fig 3/8)

Sun-orientation

Compact settlements should be located on shaded slopes
(north-sloping) and at higher levels. The general preference for the orientation
of slopes referring to sun exposure (on the northern hemisphere) is: 1st: north;
2nd: east; 3rd: south; 4th: west. This can vary in relation to the local
conditions, topography, vegetation, sun angle and exposure time. e.g. sites on
north / southeastern slopes are also acceptable. Near the equator, south slopes
are preferred over east and west slope.

Depending on how much passive heating during night and cold
seasons is required, south slopes can be advantageous. (also see Chapter 3.4)

Wind orientation

ocations are preferred where the effect of cool airflow can be
utilized and controlled. High altitudes and locations with evaporative
possibilities are advantageous. Settlements have to be properly oriented
regarding prevailing winds. Winds are more frequent and relatively cooler at
higher elevations. Blowing over a water-body can result in a drop of a few
degrees in the temperature of a wind. Wind can also be caused by specific
direction and conditions in a valley.

Fig 3/87

Location in flat regions

Compact settlements in flat areas have, in general, less natural
features, such as hill sides, slopes, and rock formations which have to be
integrated to improve the micro-climate. Such settlements should include
vegetation because the air is cooled while crossing green shaded areas. A draft
is created through cooling the hot air in the shade and by the humidity of
plants or water ponds, a phenomenon well known from traditional oasis
settlements. (also see Chapter 3.1.2.3, 3.2.2.3)

Fig 3/88

3.2.2.2 Hazards (also see Chapter 3.1.2.2)

Sand and dust storms, sand dunes

The reduction of the effects of sand storms can be achieved
through the location of settlements at higher elevations and landscaping cities
with plants and water, which lead to less sand in the air. The open space
pattern (network of streets and squares) has to be planned accordingly, e.g. an
irregular pattern to break strong winds (see Chapter 3.2.2.3). Particular
attention has to be paid to the moving direction of sand dunes, which can slowly
bury houses and entire settlements.

Floods

In desert areas, the so-called wadis (dry valleys and rivers)
can be very dangerous places because of their bad drainage and sudden inundation
in case of heavy precipitations.

Landslides

Heavy precipitations can cause landslides both at the bottom of
valleys and on slopes.

Earthquakes

Safe constructions can be in contradiction to traditional design
or climatic construction requirements, particularly in the case of simple mud
(adobe) or brick buildings.

· Plan close proximity of urban
services and daily functions within walking distance; wide roads can thus be
omitted or at least reduced.

· Avoid large open spaces within
the city where hot air can collect during the day and which are conducive to
duststorms.

· Provide ample shaded public
spaces.

· Select light colors for every
open space.

· Include green areas of plants
around and within the settlement to provide shade and cool air and to stabilize
the soil.

· Plant and cultivate xerophytes
that require little or no water.

· Integrate water bodies, which
evaporate and therefore reduce temperature.

Fig 3/89 Traditional and imported
urban patterns

Minimal sun-exposure in summer and therefore compactness and
shade are the main principals for building in hot-arid zones. Hence, compact
planning for groups of buildings is required in order to give shade to each
other and to provide a shaded network of narrow streets and small spaces in
between as patio-like areas. Arcades, colonnades, cantilevered buildings or
building components, membranes and small enclosed courtyards are traditional
responses to the climate; even larger public open spaces should be enclosed,
inward looking and shaded for most of the day. Of equal importance is natural
lighting and ventilation. Air circulation can be improved through wind
channelling in shaded narrow streets in the direction of the main wind. The
grouping of buildings and alleys or lanes should allow for proper ventilation or
even increase the airflow. The location near a water source and the
incorporation of vegetation is most important. [ 9 ]

Fig 3/90 Shady arcades

Settlement patterns and street-networks (also see Chapter
3.1.2.3)

Urban forms are not only a result of physical and functional,
but also of social and cultural factors and traditions in a region. There are
different ways of properly designing an urban form in an arid region taking into
account solar radiation and wind.

Some basic possibilities:

a) Grid diagonal to east-west axis

The grid pattern maximizes radiation throughout its straight
streets, but by orienting the grid pattern diagonally to the east-west axis, the
sun exposure and shade is better distributed on the streets; such a grid still
supports the dynamic movement of air. More important, however, is the form of
alleys and buildings. [ 5, 143 ]

Fig 3/91 Grid diagonal to east-west
axis

b) Narrow, zigzagging alleys

Winding or zigzagging narrow alleys receive minimum radiation,
reduce the effect of stormy winds, establish shaded spaces throughout the day
which provide a cool and comfortable microclimate and also stay relatively warm
during cold nights and in winter.

Fig 3/92 Zigzagging alleys

c) Blocked streets and alleys

Street orientation and housing patterns are significant and must
be planned carefully. Straight and parallel streets open the city to wind
ventilation. Storm effects can be reduced by blocking streets. Two-story
buildings with closed patios open to the sky will maximize shade, minimize
radiation, yet still retain ventilation and reduce the effects of stormy winds.
Buildings should be attached (cluster) to reduce exposed surfaces.

Fig 3/93 Blocked streets

External space design

External space design(also see Chapter 3.1.2)

The town structure and the public spaces should thus counteract
heat with a shaded and dense layout. There should be a close connection between
public spaces and residential areas. Dwelling units or groups should create
patio-like areas. Paved open spaces within a flat cityscape should be avoided or
kept to a minimum size.

Most important is the design of the whole urban configuration,
because the ratio of shaded space to space open to solar radiation affects air
temperature significantly. The temperature in and around buildings can either be
tempered or aggravated by the nature of the surrounding surface. The
temperatures shown in Fig 3/94 were recorded in a hot-dry climate when the air
temperature was 42°C [ 106 ]

Fig 3/94 Temperatures of
differently-treated surfaces

a) Street-scaping

Particular attention has to be paid to the needs of the
pedestrians, walkways and the scale of the environment. Half and full shade
protection by arcades, membranes etc., and vegetation (trees) is desirable;
exposed paved surfaces should be avoided; pools of water are beneficial.
(also see Chapter 3.1.2.3 and 3.3.2.3)

b) Landscaping with vegetation (also see Chapter 3.1.2.3)

Trees, hedges and plants in an urban context can have a dramatic
effect on the microclimate and help to tie down sand and dust [ 1 ]. As
vegetation is generally sparse, an oasis-like concentration of plant and
grass-covered areas is desirable. Nevertheless, landscaping should not always
imply the inclusion of very high water consuming lawns and grassed areas. Local
desert plants as well as rock and stone garden as well as gravel coverage should
also be considered as adequate design elements.

Fig 3/95 [ 124 ]

c) Pattern of green areas

The vegetation in and around the city promotes and controls air
movement. Apart from water areas, evaporation and cooling takes place only in
green areas. Green areas located near and in a city will therefore improve the
urban climate. The difference in temperature between green areas and built-up
land causes minute air cycles and a horizontal exchange takes place. An
arrangement of small parks and lanes could facilitate the ventilation of the
town. The wind from the countryside is encouraged to penetrate as far as
possible into the built-up area. [ 124. 134 ]

The orientation of a building is influenced by the amount of
solar radiation falling on different sides at different times. Buildings are
best arranged in clusters for heat absorption, shading opportunities and
protection from east and west exposures. Protection from solar radiation is
particularly important during times of excessive heat when there can be a
difference of as much as 3°C in air temperature in a building between the
best and least favourable orientation. The larger building dimension should face
north and south (generally, west orientation is the worst: high air temperature
combined with strong solar radiation) [ 9 ]. The optimum orientation for any
given location has to be determined in order to achieve the most satisfactory
distribution of total heat gain and loss in all seasons. At high altitude enough
heat gain for passive heating should be possible.

In general, the best orientation is: north-south with 25o south
easterly direction [ 13, 161 ]. Attention should be paid to solar radiant heat
reflected from the surroundings (topography, slopes, rocks) to the building.

Wind-orientation

Main walls and windows should face the prevailing (cool) wind
direction in order to allow maximum cross-ventilation of the rooms.

3.2.3.2 Shape and volume (also see Chapter 3.1.3.2)

The shape and volume of buildings should be compact, yet
somewhat elongated along the east-west axis; (e.g. the optimum shape is 1:1.3),
because large, compact building volumes gain less heat. In general, the optimum
shape is that which has a minimum heat gain in summer and the maximum heat gain
in winter. Under winter conditions an elongated form is ideal; under summer
conditions a square shape is better [ 9 ]. A compact patio house
type is therefore preferable. Adjoining houses, row houses, and group
arrangements (all continuous along the east-west axis), which tend to create a
volumetric effect, are advantageous, as are high massive buildings [ 13 ].
Lithospheric arrangements (subterranean) are also applicable.

Fig. 3/97 Shading of buildings and
building elements by cantilevered construction, arcades, loggias and high
building parts.

3.2.3.3 Type and form of buildings

Dense settlement patterns require a particular type of building
consisting of compact structures and forms. Subterranean spaces are also
adjusted to climatic stress. In hot-arid zones, external and internal living
spaces have to be protected against solar radiation, glare, and hot, dusty
winds. Compactness can be achieved by carpet-planning layouts with
courtyard houses or cluster settlements of high buildings to create suitable
patterns. Particular solutions may utilize underground (subterranean) buildings
or caves. Some heat gain and storage in the winter season is desirable.

Fig 3/98

The main objectives are:

· Compact and massive design,
mainly inward-facing buildings.

· Minimize surface areas and
openings exposed to the east and west sun and orient the building accordingly.

· Allow heat gain and storage in
winter.

· Group buildings closely to
each other. Especially east and west walls should be placed closely together for
mutual shading.

· Create thermal barriers
(non-habitable rooms, such as stores, toilets etc.) on the east and especially
on the west side of the building.

· Include small enclosed
courtyards with arcades, colonnades for light and air and outside day-to-day
activities. Courtyards provide shade, cool air pools, and protection from hot
and dusty winds.

· Treat the external space as
carefully as the building itself to reduce glare and reflected heat radiation.

Courtyard design

It is difficult to meet all the different functional and
climatic requirements. Regarding the volume, the patio-house is the
most suitable form and can benefit in summer from the microclimatic effects of
cool air pools that occur in courtyards. Although winter conditions in hot-arid
regions would permit an elongated house design, the heat in summer is so severe
that a compromise is required. The very old, traditional solution - particularly
for flat land - is a compact, inward-looking building with an interior
courtyard. This minimizes the solar radiation impact on the outside walls and
provides a cool area within the building. It also meets other requirements such
as safety, defense, privacy, lifestyle etc.

Fig 3/99 Schematic plan of a typical
Egyptian house built prior to 3000 BC.

In the typical oriental courtyard house, the covered terraces,
which are usually on two or three sides of the courtyard, and the identical
covered gallery on the first floor help to reduce the heat gained during the day
and provide shaded areas. The correct ratio between the height and width of the
courtyard should always allow for adequate shading, even when the summer sun is
almost directly overhead. When the courtyard is provided with water and plants,
it acts as a cooling source and modifies the microclimate accordingly.

In areas with cold nights or winters the court yard has to allow
for adequate south exposure for passive heat gain and should be equipped with
movable shading devices for the hot period.

However, the one or two storied courtyard building type cannot
always fulfill todays functional and urban planning requirements, where
high population density, economic land use, adequate car traffic, accessibility
and suitable public transportation, etc. are required.

Fig 3/100 Courtyard house with
covered galleries and an internal pool for evaporation, day and night situation

Tall buildings

In certain regions, such as mountainous and coastal areas,
(North Africa, Arabian Peninsula, etc.) high, compact buildings are the
traditional solution, having also had an important defense purpose in the past.
Cooler air from the lower floors is channeled through the building. High walls
with integrated ventilation shafts are built at the back on the shady side. In
maritime regions, large openings or bay windows for cross-ventilation are
protected with wooden screens such as Rowshans or
Mashrabiyas. (see Chapter 3.2.5.1)

Fig 3/101

Underground buildings

Underground dwellings have been known for thousands of years. At
a depth of about 2.5 m, the temperature of the earth is practically constant and
remains close to the average yearly temperature. The indoor climate of
structures built underground or covered with a thick layer of soil benefits from
the huge thermal mass of the adjacent ground and is thus not affected by hot
days and chilly nights. Structures can be carved into suitable rock formations
or may consist of a structural shell (even several floors underground), which is
mainly concrete and covered by soil. (The provision of natural lighting might
cause difficulties.)

Rules:

· Where the diurnal temperature
range is wide, but the daily average is within the comfort zone, a soil cover is
appropriate.

· Where the annual average
temperature is within the comfort zone, structures built 2-3 m underground are
suitable.

· High rooms (ceilings) are not
necessary.

· Natural lighting must also be
considered.

· Protection against surface
water (flooding) may be required. Structures within the groundwater table should
be avoided.

[ 5, 7, 9, 136 ]

Fig 3/102 Section through an
underground dwelling

Buildings in maritime, coastal regions

In certain regions, the high summer humidity in maritime areas
makes designing buildings here extremely difficult. More ventilation is required
at times and high thermal capacity structures are less effective. Tall buildings
and building components with lightweight structures which utilize the breeze for
rooms used in the daytime are good traditional solutions to reduce discomfort.
The use of high thermal capacity structures, although still useful, will not be
as effective as in other hot-dry regions. The coastal wind blowing off the sea
during the day may be utilized to ameliorate thermal conditions. On the other
hand, the nighttime wind carries hot inland desert air, possibly dust, towards
the sea, which can be very unpleasant. Protection from this wind should be
provided.

Perhaps the only solution is to provide alternative spaces: one
with high thermal capacity walls and roof, for use at night, especially during
the cooler part of the year; and one of lightweight construction, the roof
providing shade only and the facades facing and opposite to the sea being left
almost completely open. This is the best solution for daytime use, especially
during the hottest part of the year.

It is in this climate that wind catchers, scoops and wind towers
have their greatest benefit. [ 8, 9 ] (also see Chapter 3.1.3.3, 3.2.4.3,
3.2.5.2 and 3.4.3.3)

Table: The concept of alternative day and night space

Type of structure

Performance

Suitability

winter

summer

Heavy structure

Cool in daytime

night

daytime

Light structure

Cool at night

daytime

night

Room arrangements (also see Chapter 3.1.3.3. and 3.4.3.3)

The room layout depends on the building type. A courtyard design
has certain advantages. Heat-producing areas should be separated from other
areas of the house. Non-inhabitable spaces should be placed on the west side to
check the suns impact. Internal heat gain can be avoided by a functional
layout.

Bedrooms should be on the east side, and outdoor or roof
sleeping possibilities should be considered. Living rooms should be on the north
or south side. The depth of interior spaces should allow for proper natural
lighting. Nevertheless, modern floor plan requirements, multi-family housing
(high density) and different values, such as access to a view, might be in
contradiction to climatic design principles.

3.2.3.4 Immediate external space (also see Chapter 3.1.3.4)

The walls of houses and courtyards, cantilevered building parts
and plants should provide shade to outdoor living areas. Half and full shade
protection by arcades or loggias, membranes and trees is desirable; exposed
paved surfaces should be avoided; pools of water are beneficial for cooling.

3.2.4 Building components (also see Chapter 3.1.4)

The main points:

· Control of heat transfer
through thermal storage and time lag by proper construction and materials
· Thermal insulation to reduce internal heat
gain.· Reflectivity and emissivity to
re-radiate heat.· Control of air movement

Building materials

The comfort of people inside the buildings depends largely on
the thermal properties of the outer and inner walls and the roof. Depending on
the function of the building components specific insulating and/or thermal
storage qualities are required. (Basic explanations see Chapter 3.1.4)

Buildings in hot-arid zones are traditionally constructed with
thick walls and roofs and with very small openings. An internal thermal storage
capacity is very important to decrease the temperature variations and to make it
possible to profit from an increased night ventilation by storing the cool
of the night until the day during summer. The best materials are those
that do not conduct heat.

Fig 3/103 Heat flow in daytime and at
night

Sun-dried earth brick is one of the poorest conductors of heat,
partly because of its very low natural conductivity and partly because mud is
structurally weak and necessitates thick walls. Yet thick mud bricks are not a
perfect means of keeping cool; they retain heat for a long time. Therefore, it
is important to calculate and plan the proper time lag. A big thermal mass can
keep cool during the daytime and not be too cold at night. (see example Chapter
4.4) High heat capacity walls are essential. The traditional principle is to
shelter behind very thick mud walls by day, and to sleep on the roof under a
tent at night. [ 122 ]

Construction concepts and details

The different building components require adequate design and
material properties to act as a balanced system.

· Thermal insulation is
important to suppress surface temperature variations, but is only applicable in
connection with adequate inner ventilation and cooling means or in combination
with light structures (Insulation can also reduce necessary heat loss at night).
Roof insulation is especially important in decreasing summer temperatures. The
outside application of insulation is preferable because the structure and the
construction materials are less exposed to thermal stress, and the storage
capacity of a heavy structure material helps to balance the inner temperature.
The additional needed skin for the building or roof must protect the insulation
against damage by physical, mechanical forces, and should be of a hard material.
The required insulation value depends on the sun exposure. (see Chapter 3.1.4)

· Time lag properties of
building parts and its materials should be used for energy storage and
temperature exchange between day and night. Necessary time lags for internal
heat balance are: Walls, east: 0 hours; south: 10 hours.; west: 10-hours.;
north: 10 hours or no lag; roof: 12 hours [ 13 ] (also see Chapter 3.1.4)

· Shading devices, such as a
heavily ventilated double roof, and radiation reflection by a white surface are
necessary to decrease heat gain from solar radiation - mainly through the roof -
during the hot period.

· External colors are required
as a combination of high reflectivity of solar radiation and high emissivity of
infrared radiation to the cool sky at night: white, non-shiny surfaces, avoid
all dark colored surfaces. White paint has a high reflection ratio on sun
exposed surfaces. Dark absorptive colors are usable where reflection towards the
interior should be avoided (such as under eaves). Deep-set surfaces can be
dark-colored for winter radiation absorption. Bright color contrasts should be
in agreement with the general character of the region. [ 13 ]

· Internal colors, such as
cool and bright colors can be used psychologically as a cooling
contrast to intense outdoor heat and to distribute natural light for deep room
arrangements.

3.2.4.1 Foundations, basements and floors (also see Chapter
3.1.4.1)

The ground is a valuable means of heat absorption; therefore the
building should have maximum contact with the ground. Ground floors should be
solid and built directly on to the ground or into the ground with heat absorbing
materials (stone, adobe, earth, high density burnt clay or cement products)
Ground floors should not be suspended and on no account be built on stilts.
Flooring materials should be of high thermal conductance. The ground near the
building should be shaded during the day, but fully exposed to the night sky, so
that the emission of radiant heat is not obstructed. [ 8 ]

Fig 3/104

3.2.4.2 Walls (also see Chapter 3.1.4.2)

During the hot season, walls of daytime living areas should be
made of heat-storing materials; walls of rooms for nighttime use should have a
light heat capacity. East and west walls should preferably be shaded. High
reflective qualities are desirable for both thermal and solar radiation. [-13 ]

In regions with a less extreme diurnal temperature range and
where the night temperature does not fall below comfort zone, the internal walls
and intermediate floors should have large thermal masses, whilst the outer walls
and roof need a high resistive insulation and reflectivity [-8 ]. Double walls
with insulation in between are a suitable solution. (multylayer construction,
see Chapter 3.1.3)

In regions with large diurnal temperature ranges and night
temperatures below comfort level, inner and outer walls and - especially in the
absence of a ceiling - roofs should possess a large thermal capacity with an
appropriate time lag to balance temperature variations. To achieve this they
must be constructed of heavy materials. The use of exterior or interior
insulation has to be considered carefully and its suitability depends on the
particular requirements and technical possibilities.

3.2.4.3 Openings and windows (also see Chapter 3.1.4.3)

Openings and windows are necessary for natural lighting and
ventilation, but heat gain in summer should be minimal. During the daytime, the
absence of openings would be desirable, especially on the west side; or the
openings should be as small as possible and be shielded from direct radiation
and located high on the walls to protect from ground radiation [ 13 ]. At night,
the openings should be large enough to provide adequate ventilation for the
dissipation of heat emitted by the walls and the roof. Hence larger openings
should be closed during the day with insulated shutters and opened at night.
Such systems are not always reliable because they require the attendance and
readiness of the inhabitants. Other considerations such as desired privacy and
safety may prevent the correct use of a system with shutters.

Appropriate natural lighting is important. The depth of rooms
and the size of windows have to be coordinated. Glare of direct natural lighting
can also be avoided by the use of internally reflected light.

Fig 3/105 Indirect natural light

Orientation and size of openings

Main openings should face north and south, but the latter should
be shaded either by shading devices, roof overhangs or by deciduous trees. The
size of the windows on the west and east sides should be minimized in order to
reduce heat gains into the house in the early morning and late afternoon, or
also be protected by particular shading devices. A moderate, south-facing glass
area catches the solar radiation during the cold season, but should not be
affected by direct radiation during the summer.

Window glass (also see Chapter 3.1.4.3)

Generally, single glazing is sufficient. Insulating and special
heat-absorbing and heat-reflecting glass is basically only suitable for
air-conditioned buildings. Generally, single glazing is sufficient. Tight
closing joints and window profiles are important to prevent the penetration of
hot air, sand, dust and insects. (also see Chapter 3.4.4.3)

Placement of openings

Windows and other openings must be placed in suitable positions
relation to the prevailing (cool) breeze to allow a natural airflow through the
building, to achieve air movement across the body for evaporative cooling and
air changes for driving out excess heat. An internal draft (cross-ventilation)
can be channeled by louvres set in an upward position towards the ceiling or in
a horizontal position towards the human body. Outlet openings should be located
at a high level where hot air accumulates.

In buildings in coastal areas, openings for cross-ventilation
should be equipped with movable shutters. Because of the hot land wind which
occurs at night, openings facing the inland direction should be closable. [ 1 ]
(also see Chapter 3.1.5.2)

For comfort, ventilation openings should be at the level of the
occupants. High openings vent the hot air collecting near the ceiling and are
most useful for convective cooling.

Fig 3/106 Placement of openings

3.2.4.4 Roofs(also see Chapter 3.1.4.4)

Different forms of roofs are possible or can be traditionally
applied, the latter mainly determined by local materials and technical means. In
hot-arid regions the vault, the dome and the flat roof are the traditional roof
shapes. The common construction method of today, a 10 to 15 cm exposed concrete
is the worst possible solution, because the inner surface temperature can reach
up to 60°C, which remains till late in the evening.

As the roof is the most critical part, high solar reflectivity
and emissivity for long-wave radiation are essential, as well as thermal
insulation and/or adequate time lag. Outside application of insulation is
preferable for reasons mentioned earlier, but needs an additional, robust skin
which protects the insulation from damage.

The rounded form of a hemispherical vault (dome) has a larger
surface area than its base. Solar radiation is thus diluted and re-radiation
during the evenings is also greatly facilitated. [ 9 ]

Fig 3/107 Example of dome and vault
structures

The flat roof is practical in areas where it seldom rains. It is
also a good reflector and re-radiates heat efficiently, especially if it
consists of a solid, white painted material. (see Chapter 3.1.5, 3.2.5.2,
3.2.5.3) [ 13 ] High solid parapet walls along the edge of the roof can on the
one hand provide daytime shade and privacy, but can have the disadvantage of
creating an undesired stagnant pool of hot air. The construction and exact
placement of parapet walls should therefore be carefully examined. [ 8 ]

Fig 3/108

A separate roof and ceiling are still today less common in
hot-arid regions, whereas they are the obvious solution in warm-humid climates.
This efficient, but expensive solution (pitched or flat ventilated double roof)
contrasts with the traditional form of most desert buildings. However, the
sloping roof with wall shading overhangs and a well-ventilated space between
roof and ceiling appears to be an appropriate, contemporary solution. [ 147 ]

Fig 3/109

If it is used, the material of the roof should be light and the
ceiling material should be massive. The air enclosed in a double roof, or
between the roof and ceiling, may reach a very high temperature. This can be
avoided by ample ventilation of the roof space by openings facing the prevailing
breeze. In addition, roofs (slopes) should be orientated towards the prevailing
breeze and any obstructions which would prevent the airflow next to the roof
surfaces should be avoided.

Fig 3/110 Ventilated double roof
with heavy ceiling

A somewhat less effective but also less expensive construction
would be a simple ceiling with a ventilated roof space (also only common in
warm-humid climate zones). A shaded, ventilated roof is applicable primarily
over rooms used at night.

Fig 3/111 Ventilated double roof
with light ceiling

Sloped roofs could also provide cold airflow towards a
courtyard. A membrane covering the courtyard in the daytime allows retention of
cool air and provides shade, but needs attendance by the inhabitants.

The efficiency of the central courtyard is increased by
stretching a curtain across the courtyard early in the morning during the summer
months to trap the cool air. In the evening, this is removed to maximize the
night radiation potential. [ 106 ]

Fig 3/112

3.2.5 Special Topics

3.2.5.1 Shading devices (also see Chapter 3.1.5.1)

In hot-arid zones, shading of the direct suns radiation
and its reflection by surroundings is essential; diffuse radiation is less of a
problem. Shading can be provided by different means, such as placing buildings
closely together, the shape of the building itself (overhangs etc.), vegetation
such as deciduous trees, or attached, special shading devices.

In hot maritime regions, the traditional mashrabiyas
or rowshans are common. These projecting, screened (bay) windows or
non-projecting screened windows consist mainly of wooden, shading screens over
large openings and allow cross-ventilation as well as the passage of daylight
while preserving family privacy. Some contain evaporative cooling means such as
an earthenware water pot.

Similar devices can be designed by contemporary means (see
Chapter 3.1.5.3).

Ventilation is essential and must be regulated to achieve the
highest efficiency in keeping hot (and dusty) air out during the daytime, and
cooling the thermal mass at night by air movement; if possible together with
outside vegetation. Ventilation can only reduce temperatures higher than the
outside air temperature. However, if the air is very dry, any breeze also helps
to evaporate sweat and thus to cool the body. High rooms promote air circulation
and increase the distance to a radiating ceiling. A low ventilation rate during
winter decreases the temperature variation and thus raises night temperatures. A
high night ventilation rate in combination with an internal thermal storage
capacity is preferable during summer.

During the daytime, openings should be closed and shaded and
ventilation kept to the absolute minimum necessary for hygienic reasons.
Openings should be placed according to the prevailing winds and allow
cross-ventilation. Air intake openings should be located so that the coolest and
most dust-free air is taken and, if necessary, the air can be conveyed to the
points in the building where it is needed. Thus the cool conditions existing at
dawn can be maintained inside the building for the longest possible period.
Internal heat sources should, if possible, be isolated and separately
ventilated.

Electric fans (ceiling mounted etc.) may be used where little or
no air movement occurs. (see Chapter 3.1.5.3)

Windcatchers (also see Chapter 3.1.5.2)

Windcatchers are a significant feature in the traditional
structures to ventilate and cool buildings in hot desert and hot, coastal
regions. Wind pressure forces air down the wind catcher. Air circulation inside
the building is achieved if there are openings on the opposite side allowing
suction of inner air by lower pressure.

Depending to the region, they have a variety of forms, details
and ways of functioning, and are known in the Middle East as malqaf
and/or badgir [ 122, 149, 155 ]

a) Roof windcatcher

One kind of windcatcher (also called wind chimney)
is built onto the roof. In some places the catchers are unidirectional and
orientated to catch favourable winds or are facing away from it to draw cool air
from the court yard through rooms, and expel stale air and smoke. By change of
wind they are anticipated to reverse their function.

In other places pivoted scoops and multidirectional wind towers
utilize winds from any direction. Generally, windtowers are square in plan and
have four internal shafts.

The principle involved is to catch an unobstructed breeze at a
high level and channel it to areas in the bottom parts of the building. The
increased air-velocity supports perspiration and is thus cooling. The ducts are
preferably built in a massive way to absorb the heat of the incoming air and not
exposed to solar radiation (e.g. northern wall), to enhance efficiency. In
addition they should be equipped with evaporative cooling means, such as porous
water jugs, moist matting, wet charcoal etc., to achieve efficient cooling (also
see Chapter 3.1.5.3).

Fig 3/114 Unidirectional roof
windcatcher

The inlet of the catcher should have a shutter to regulate the
air movement and to protect against too cold or too hot air and against sand. [
9 ]

In the Middle East, wind catchers can provide sufficient
ventilation and cooling during approximately six months of the year for
comfortable inner climatic conditions of todays comfort requirements,
without additional devices or the use of mechanical cooling or heating systems.
[ 155-]

Structurally integrated wind catchers or scoops and air ducts
are a special kind of vents and selective ventilators. A recessed, horizontal
niche on the external wall, e.g. on the floor level and in the roof parapet,
creates a slot between two vertical, structural posts. These mid-wall or parapet
wind intakes or series of them may allow for enough cross-ventilation through
the internal spaces in humid weather, while preserving visual privacy. Shutters
are necessary to control the air movement. Vertical air shafts integrated into
the wall provide air circulation within the building. [-155, 166-]

Fig 3/116 Mid-wall and parapet
windcatchers

Solar chimneys and induction vents(see chapter 3.1.5.2)
These methods can also be applied in hot-arid regions.

Forced ventilation

Electric ventilators or fans represent simple active devices.
They may be placed directly in the outer wall or combined with an air duct
system.(also see Chapter 3.1.5.3)

3.2.5.3 Passive cooling means(also see Chapter 3.1.5.3)

Cooling means should be integrated into the general ventilation
concept of a building. Cooling can be achieved by the evaporation of water. The
dryer the air, the greater is the cooling potential.

A courtyard house with a dry, hot yard and a cool yard with
vegetation and a pool represents a good example of such a ventilation concept. A
draft which passes through an evaporative cooler before entering the main rooms
is created by the two yards.

Fig 3/117 Cooling system of a
courtyard house

External cooling

External cooling External cooling through humidification can be
achieved by keeping the surfaces of roofs and / or walls moist. (e.g. lawn
sprinkler) The surface temperature can be reduced by up to 30°C. However,
the water consumption is excessive.

Fig 3/118 Sprinklers on the roof

Evaporative coolers

Air cooling and humidification or simple air conditioning
devices are important means of internal cooling. Warm and dry air passing over
water is cooled by evaporating the water. Evaporative coolers have a limited
effect and should only be used in relatively dry climates.

Fig 3/119 Evaporative cooler
combined with a wind tower [-122 ]

a) Moist matting

An open weave matting of vegetable fibre (straw) is stretched on
a wooden frame and is kept moist. The matting should be as fine as possible,
placed in front of windows and in the path of the natural airflow. The natural
airflow should not be reduced and can also be supported by a fan. The damp
matting humidifies and cools the air as well as filters out the dust. [ 136 ]

Fig 3/120

b) Earthenware pots

Another simple system entails the use of large, porous
earthenware pots filled with water which seeps through the walls of the pot
moistening the outside and, as it evaporates, cools the passing air. [ 9 ]

Fig 3/121

c) Wet charcoal and water pools

In wind catchers, beds of wet charcoal over which the air passes
before entering the room, are sometimes used. The same principle can be applied
by channelling breezes over pools or water sprays before they enter buildings. A
spray pond is more effective than a still pool of the same size and has the
additional advantage that the air is not only cooled, but also cleaned by
binding the dust particles. Availability of water and maintenance aspects should
not be neglected. [ 122 ]

Fig 3/122

Roof pond (also see Chapter 3.1.5.3)

A water body covering the roof functions similarly to a soil
cover: it minimizes the diurnal temperature range. It is a technically demanding
and expensive solution. It also requires the daily attention of the users and is
not very suitable for hot-arid regions of the Third World.

Thermal walls and solar collectors (also see Chapter
3.1.5.3)Solar walls are usually used to heat buildings and hence less
suitable for hot-arid zones (see Chapter 3.4.5.3). They can, however, also be
used as a cooling device.

A wall exposed to the sun can be built in the form of a solar
collector and used to create a draft. The air warmed up by the solar collector
creates a buoyancy which moves the air in the room. The air entering from there
is cooled down by an absorber and perhaps additionally by an evaporative cooler.

Fig 3/123

3.2.5.4 Active devices (see Chapter 3.1.5.4)

Active devices, such as air conditioners, are often unavoidable
and require a different building construction. Many passive means of
climatization, however, are also beneficial in that they drastically reduce
running costs. With the increased possibilities for using solar energy, active
devices may become the means of the future.